1. Field of the Invention
The present invention is related to the electromechanical field.
The invention will find a very particular, but in no way restrictive, application in the field of the electro-dynamic transducers.
These electro-dynamic transducers are generally used as loudspeakers, geophones and microphones.
The present invention relates to a moving part for an electro-dynamic transducer, comprising at least one mandrel supporting a winding of at least one coiled wire.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
In a known way, an electro-dynamic transducer comprises a moving part generally formed of a coil of copper or aluminum wire wound about a mandrel, the latter being made out of a flexible and light material, such as aluminum paper or imide-based polymeric films, such as KAPTON™. Furthermore, within the framework of a loudspeaker, a moving part comprises a diaphragm, commonly referred to as “membrane”, connected to an end of its mandrel. The latter serves as a support for the wire of said coil, is in the form of a cylindrical tube. The unit comprised of a coil and a mandrel is mounted movably in the axial direction of said coil within a space referred to as “gap” through which passes a permanent magnetic field, so that, when a current flows through the wire of the coil, the latter moves within the gap. In other words, this system permits to convert an electric current into mechanical energy, or vice-versa.
It should be noted that said permanent magnetic field is generated by a magnet.
The displacement of the moving part induces vibrations of this diaphragm, generating acoustic waves depending on the current flowing through said coil.
In particular, a moving part oscillates inside a guiding sheath. The latter has a cylindrical shape, surrounding part of said moving part and inside which said magnetic field is generated. The evolution of the technique tends to improve the efficiency and the behavior of the dynamics of such a moving part.
Therefore, the mandrel and the membrane are made out of the lightest and thinnest possible materials and are shaped in compliance therewith. Namely, the mandrel is in the form of a cylinder emptied in its central portion, onto which is peripherally wound the wire of said coil.
The reduction in weight of the mandrel and the diaphragm often results into a reduction of their rigidity; this reduction of the rigidity can result into normal resonating modes in the pass-band of the loudspeaker. Non-linearities result from these normal modes, which affect the frequency response of the electro-dynamic transducer. These non-linearities generate harmonic and non-harmonic sound distortions, which are in particular unpleasant when listening.
In addition, the energy of the normal modes, which is not restituted in acoustics is stored during the excitation of the moving part, then restituted as soon as the excitation stops, leading to a tailing phenomenon, to the detriment of the quality of the sound produced.
In order to improve the efficiency, increasingly lighter, but also more rigid materials are used. For more efficiency against the normal modes, some parts are also thicker, in order to increase their rigidity, and are therefore heavier, in contrast to the previously contemplated solutions for improving the efficiency.
Depending on the guiding used for the displacements of the moving part, the design of the mandrel supporting the coil often gives rise to a long and hollow part, which, on the one hand, produces an extra weight and, on the other hand, possesses normal modes of vibration in the pass-band of the electro-dynamic transducer.
In addition, the high excitation of such a mandrel can cause its deformation, largely altering its lifetime and its capacities.
A known solution is described in JP 2001 359193 relating to a moving part for an electro-dynamic transducer comprising, on the one hand, a diaphragm that constitutes emitting means and, on the other hand, a two-part member constituting the support for the coil. The latter transmits its vibration to the diaphragm through said member, which then acts as a tube for transmitting the vibration of the coil. Because of its shape and its positioning, this tube does not emit any useful acoustic wave, since it “splits” the air during its displacement, but, on the other hand, generates parasitic vibrations and, as a tube connecting the coil to the diaphragm, it possesses modes of vibration that are specific to it. Only the diaphragm causes the air to move within such a part. Thus, the mandrel only serves as a support and does not emit at all. Such a moving part falls into the present technical attempt to control the normal modes of vibration of the support of the coil, without being capable of eliminating them.
Another document, U.S. Pat. No. 6,587,570, describes a similar system with a tube supporting a diaphragm and having parasitic normal modes of vibration.
Another solution is described in U.S. Pat. No. 5,647,014, relating to a moving part comprising a tube that encloses the coil at the periphery within a groove. Said tube does not include any emitting means and serves only as a support for the coil.
Furthermore, from Guy LEMARQUAND's publication, with the title “Mechanical Properties of Ferrofluids in Loudspeakers”, is known a movable part for an electro-dynamic transducer comprising a mandrel that is in the form of a piston, at the periphery of which is provided for a groove receiving said coiled wire in winding, which then serves as a support for the coil. In addition, said piston slides through ferrofluids within a longitudinal recess that acts as a guide. Said piston emits sound vibrations by moving.
However, such a piston remains heavy to be moved for a reduced travel distance within said recess, thus limiting the emitted vibrations, which are furthermore disturbed by the length of the piston.
Such devices are therefore not fully satisfactory and do not permit to avoid the parasitic vibrations.